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Plaster/Silica Mixtures for Mold Making

The traditional mold material used for kiln casting is a 50/50 mixture of plaster and ground silica. This produces a mold that is inexpensive and will work fairly well for small projects.  For larger items it's often better to reinforce the plaster/silica or even use an investment other than plaster/silica.

Normally, the plaster used can be any of the commonly available pottery plasters.  Even plasters such as Plaster of Paris will work well for most purposes.

Silica, which is a hazardous material that can cause silicosis (a lung disease), is generally used in powder form.  200 mesh works well.  In some places ground silica is also known as flint or silica flint.

Although the traditional mixture uses equal parts of each of these two ingredients, the ingredients do not generally need to be measured precisely (especially when used in small molds).  It's acceptable to add more silica than plaster -- adding up to two times as much silica will leave a softer mold that can easily be carved and shaped after it sets. 

Click here to go to part two of this tip, which deals with mixing plaster and silica together.

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Copyright 2005 Brad Walker.  All rights reserved.

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